Hello,
I am setting up a system for a master gain control for a preamp. Digital potentiometers will be used to set the gain. There will be a traditional log volume pot (digital) on the input. In the feedback loop there will be another digital pot that varies to set the gain from about unity up until 20db max. Should I use a linear or a log pot in the feedback loop?
I am setting up a system for a master gain control for a preamp. Digital potentiometers will be used to set the gain. There will be a traditional log volume pot (digital) on the input. In the feedback loop there will be another digital pot that varies to set the gain from about unity up until 20db max. Should I use a linear or a log pot in the feedback loop?
In a simulator the gain is non-linear with a linear pot. It resembles an exponential increase. At first there is barley any change in gain but takes off in the end. With a log pot the effect is exacerbated even more.
The picture shows an example of this situation using a linear pot. What I am trying to do is create a variable gain control, in an opamp feedback loop, to adjust the master gain of a line stage.
The intent of this exercise is to compensate for signal level disparities in various audio components that supposedly put out a "line level" signal. One line component may put out a lower or higher signal relative to the nominal line level rating of .447 volts peak. Also, different power amplifiers want different signal levels. So to ensure an even balance between all stereo components, this master gain is adjusted and stored in memory for each selected component.
Based on the chart, is a linear pot the way to go here?
The picture shows an example of this situation using a linear pot. What I am trying to do is create a variable gain control, in an opamp feedback loop, to adjust the master gain of a line stage.
The intent of this exercise is to compensate for signal level disparities in various audio components that supposedly put out a "line level" signal. One line component may put out a lower or higher signal relative to the nominal line level rating of .447 volts peak. Also, different power amplifiers want different signal levels. So to ensure an even balance between all stereo components, this master gain is adjusted and stored in memory for each selected component.
Based on the chart, is a linear pot the way to go here?
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Hi,
Seems no-one has explicitly stated :
The reason power amplifiers are driven by a passive pot if fitted with
a gain control are its gain and phase margins which cannot cope with
adjusting the gain via the feedback loop.
It will either work fine - which means its severely overcompensated
for unity gain - or it will oscillate at any gain below the nominal gain
margin for the amplifiers designed nominal gain, power amplifiers
oscillating is not good for the amplifier or expensive speakers.
The same arguments apply to line level amplifiers. One of the reasons
active tone controls got a bad name is you have to slug the line
amplifier to unity gain stability (usually) for full treble cut.
No real problem with active volume control if gain is not involved, just
attenuation, and D.Self comprehensively covers the options for that.
rgds, sreten.
Seems no-one has explicitly stated :
The reason power amplifiers are driven by a passive pot if fitted with
a gain control are its gain and phase margins which cannot cope with
adjusting the gain via the feedback loop.
It will either work fine - which means its severely overcompensated
for unity gain - or it will oscillate at any gain below the nominal gain
margin for the amplifiers designed nominal gain, power amplifiers
oscillating is not good for the amplifier or expensive speakers.
The same arguments apply to line level amplifiers. One of the reasons
active tone controls got a bad name is you have to slug the line
amplifier to unity gain stability (usually) for full treble cut.
No real problem with active volume control if gain is not involved, just
attenuation, and D.Self comprehensively covers the options for that.
rgds, sreten.
Hi Guys
Self uses the Baxandall active volume circuit in his preamp, as this provides exactly the gain range requested above and uses a linear pot. It is easier to make multi-section pots track well if they have a linear taper. See the Small Signal Circuit Design handbook. There is no problem compensating this circuit as it is built around a 5532, as is usual.
Rod Elliot also shows the identical Bax circuit using a fet opamp and higher component values. This keeps the variable input impedance of the circuit from dropping too low to be easily driven by a signal source.
The circuit is inverting, which allows it to have less than unity gain. You could build it using discretes instead of ICs as someone else has done on another thread here.
Most power amp circuits are optimised for gains above a certain value, so making their gain variable can be problematic as Andrew and others have mentioned. It is possible with some circuits and certain compensation schemes to make the comp variable with the gain.
Have fun
Kevin O'Connor
Self uses the Baxandall active volume circuit in his preamp, as this provides exactly the gain range requested above and uses a linear pot. It is easier to make multi-section pots track well if they have a linear taper. See the Small Signal Circuit Design handbook. There is no problem compensating this circuit as it is built around a 5532, as is usual.
Rod Elliot also shows the identical Bax circuit using a fet opamp and higher component values. This keeps the variable input impedance of the circuit from dropping too low to be easily driven by a signal source.
The circuit is inverting, which allows it to have less than unity gain. You could build it using discretes instead of ICs as someone else has done on another thread here.
Most power amp circuits are optimised for gains above a certain value, so making their gain variable can be problematic as Andrew and others have mentioned. It is possible with some circuits and certain compensation schemes to make the comp variable with the gain.
Have fun
Kevin O'Connor
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